This invention is concerned with optical techniques for imaging a three-dimensional object in a two-dimensional format.
The general principle of a chirped radar involves sending out a laser beam whose frequency is linearly chirped in time at a rate given by S=.DELTA.f/.DELTA.t. When this beam strikes an object it is reflected and arrives at a receiver at a later time. Due to the chirp in the laser frequency, the instantaneous frequency from the laser and that of the beam returning from the target will be somewhat different. This frequency difference is proportional to the total path of the light beam to and from the target. For a distance L to the target, the round trip time is given by T=2L/c, where c is the speed of light. Therefore, if the returned beam is mixed with the instantaneous output of the laser (or with a reflection of the laser beam from a reference standard), a beat frequency indicative of the distance L will be observed. The beat frequency f.sub.o is given by f.sub.o =2LS/c. Measurement of the beat frequency over a two-dimensional image will thus provide a map of the depth information for the imaged object in terms of variations in L. Very accurate profiling measurements can be carried out using this chirped laser beam interferometry. Even though the depth information is known, however, from the beat frequency profile over the object, displaying this information in real time requires frequency measurements over numerous pixels (512.times.512, for example). The output of this information can be time consuming, even with fast electronic processing, because in many cases the beat frequencies will be small, i.e., on the order of a kilohertz. The measurement of such frequencies would require on the order of milliseconds per pixel.
Sometimes it is desirable that this three-dimensional data (the two-dimensional image information plus the beat frequency profile) be observed on a television screen (for industrial inspection, for example). The process of visually observing the beat frequency profile requires transfer of the frequency information to a television screen and modulating the intensity of each pixel by that information.